You need to give more facts, leave why?, because you and other employees, as a group, were complaining about work related issues? I assume that this is a private employer, that there is no union, and that you have no individual employment contract for purposes of this answer.
The National Labor Relations Act (NLRA), a Federal law, protects "concerted protected activities" of employees. Let me give you an example. You go to your boss alone and complain about "wages, hours, or other terms or conditions of employment" , lets say you ask for an increase in wages, you are paid 13/hr you ask to be paid 14/hr, as an example. The boss fires you for asking for an increase. Do you have a case? No, because in that example you complained alone , and since you are an employee "at will" you can be fired for any reason, good, bad, just, unjust, or no reason at all , as long as the reason is not illegal ( ex: race, national origin, disability, age, sex, etc)
Now lets say that you and your co-worker Jane Doe, went together and asked for a raise and you , or both of you, get fired. That WOULD be illegal under the NLRA, because when you acted together you engaged in "concerted protected activities" under that law, and that law prohibits employers from retaliating against employees for acting in concert.
So, when you say that the boss told "us to leave if we werent happy...", it seems that more than one employee was involved and that the issue might deal with "wages, hours, or other terms and conditions of employment". If so, and if this was a firing ( or suspension or other adverse employment action) then your boss possibly violated the NLRA.
You need to talk to an employment lawyer in your state to look at your specific situation . A good source to find on one is the National Employment Lawyers Association ( google NELA), they have a feature on their web site to find employment lawyers that represent workers in your state. Good luck
Legal disclaimer: This response is meant to be information only and should not be considered to be legal advice. This information is not meant and should not be construed to be the formation of an attorney client relationship. Employment laws may vary by state and you should contact an attorney in your state to see if a state law may be applicable to your situation.